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The Formation of a Disk Galaxy Within a Growing Dark Halo

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The Formation of a Disk Galaxy Within a Growing Dark Halo A&A 399, 961–982 (2003) Astronomy DOI: 10.1051/0004-6361:20021842 & c ESO 2003 Astrophysics The formation of a disk galaxy within a growing dark halo M. Samland1 andO.E.Gerhard1 Astronomisches Institut der Universit¨at Basel, Venusstrasse 7, 4102 Binningen, Switzerland Received 3 July 2002 / Accepted 29 November 2002 Abstract. We present a dynamical model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new three-dimensional chemo-dynamical code, including dark matter, stars and a multi-phase ISM. The simulations start at redshift z = 4.85 with a small dark halo in a ΛCDM universe and we follow the evolution until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The metal enrichment history in this model is broadly consistent with the evolution of [Zn/H] in damped Lyα systems. The galaxy forms radially from inside-out and vertically from halo to disk. As a function of metallicity, we have described a sequence of populations, reminiscent of the extreme halo, inner halo, metal-poor thick disk, thick disk, thin disk and inner bulge in the Milky Way. The first galactic component that forms is the halo, followed by the bulge, the disk-halo transition region, and the disk. At redshift z ≈ 1, a bar begins to form which later turns into a triaxial bulge. Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. The bulge in the model consists of at least two stellar subpopulations, an early collapse population and a population that formed later in the bar. The initial metallicity gradients in the disk are later smoothed out by large scale gas motions induced by the bar. There is a pronounced deficiency of low-metallicity disk stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge and inner disk (“G-dwarf problem”). The mean rotation and the distribution of orbital eccentricities for all stars as a function of metallicity are not very different from those observed in the solar neighbourhood, showing that early homogeneous collapse models are oversimplified. The approach presented here provides a detailed description of the formation and evolution of an isolated disk galaxy in a ΛCDM universe, yielding new information about the kinematical and chemical history of the stars and the interstellar medium, but also about the evolution of the luminosity, the colours and the morphology of disk galaxies with redshift. Key words. galaxies: formation – galaxies: evolution – galaxies: stellar content – galaxies: structure – galaxies: kinematics and dynamics – galaxies: ISM 1. Introduction disagreement with the flat dark matter density distributions in- ferred from observations in the centres of galaxies (Salucci & During the last decade, significant progress has been made in Burkert 2000; Blais-Ouellette et al. 2001; Borriello & Salucci understanding cosmic structure formation and galactic evolu- 2001; de Blok et al. 2001). Secondly, the specific angular mo- tion. With high-resolution cosmological simulations, the for- ff menta and scale lengths of the disk galaxies in the cosmological mation of dark halos in di erent cosmologies has been studied simulations are too small compared to real galaxies (Navarro in detail (e.g. Navarro et al. 1996; Moore et al. 1998; Col´ın et al. & Steinmetz 2000b). This may be partially a numerical prob- 2000; Yoshida et al. 2000; Avila-Reese et al. 2001; Jenkins lem of SPH simulations, but is more likely due to overly ef- et al. 2001; Klypin et al. 2001) to determine the large-scale ficient cooling and strong angular momentum transport from mass distribution, the halo merging histories, the structural pa- the baryons to the dark halos during merger events (Navarro & rameters of the dark halos, and finally the formation of galaxies Benz 1991). Thacker & Couchman (2001) and also Sommer- inside these dark halos (Steinmetz & M¨uller 1995; Kay et al. Larsen et al. (1999) showed the angular momentum problem 2000; Navarro & Steinmetz 2000b; Mosconi et al. 2001; Pearce in CDM simulations may be resolved by the inclusion of feed- et al. 2001). back processes. Thirdly, the cold dark matter (CDM) simula- In many respects, these simulations are in agreement with tions predict a large number of satellite galaxies in the neigh- ffi observations, but there remain a number of di culties. Firstly, bourhood of giant galaxies like the Milky Way Galaxy, but this the universal cuspy halo density profiles (Navarro et al. 1997; is not observed (Moore et al. 1999). Moore et al. 1999; Fukushige & Makino 2001), while in agree- The cosmological simulations are used to describe the ment with observations of galaxy clusters, are in apparent large-scale evolution of the dark matter distribution from Send offprint requests to: M. Samland, the primordial density fluctuations to the time when the e-mail: [email protected] dark halos form. It is inevitable in such simulations that Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021842 962 M. Samland and O. E. Gerhard: Formation of a disk galaxy structures on galactic and sub-galactic scales are not well- discussed in Sect. 5. A summary and a short outlook conclude resolved. Furthermore, it is difficult to incorporate the detailed the paper (Sect. 6). physics of the baryonic component, that is of the multi-phase interstellar medium (ISM), and the feedback processes between 2. Observations of the galaxy formation process stars and ISM. These are serious drawbacks, because feedback from stars can prevent a proto-galactic cloud from rapid col- Galaxy evolution models can give a simplified description of lapse and it can trigger large scale gas motions (Samland et al. the evolution of real galaxies and can make predictions that can 1997). Both alter the galactic formation process significantly. be tested by observations. The high redshift observations show Complementary ways to circumvent at least some of that the first proto-galactic structures form at redshift z > 3 these problems are (i) to combine the large-scale simulations (e.g. Clements & Couch 1996; Dey et al. 1998; Steidel et al. with semi-analytical galaxy models (Guiderdoni et al. 1998; 1999; Manning et al. 2000). Surveys of high redshift galax- Somerville & Primack 1999; Boissier & Prantzos 2000; Cole ies reveal a wide range of galactic morphologies with con- et al. 2000; Kauffmann & Haehnelt 2000), or (ii) to simulate siderable substructure and clumpiness (Pentericci et al. 2001). the formation of only single galaxies or small galactic groups At redshift z ≈ 1−2, the formation of galactic components using either cosmological initial conditions or a simplified, (spheroids, halos, bulges and disks) seems to take place (Lilly but cosmologically motivated model of the dark matter back- et al. 1998; Kajisawa & Yamada 2001). However, the spatial ground (Navarro & Steinmetz 1997; Berczik 1999; Hultman resolution of these high and intermediate redshift surveys (e.g. & Pharasyn 1999; Sommer-Larsen et al. 1999; Bekki & Chiba Abraham et al. 1999; van den Bergh et al. 2000; Ellis et al. 2001; Thacker & Couchman 2001; Williams & Nelson 2001). 2001; Menanteau et al. 2001) is still too low to measure more While the semi-analytical models are advantageous for inves- than global galactic properties (e.g. asymmetry and concentra- tigating the global properties of galaxy samples, the small- tion parameters). There are indications that most of the mas- scale dynamical models provide information about the detailed sive galaxies form before or around z = 1 (Brinchmann & Ellis structure of galaxies, the kinematics of the stellar populations 2000), and especially that the giant ellipticals form at redshifts and the ISM, and the star formation histories. The dynam- z > 1 (Best et al. 1998), (but see van Dokkum et al. 1999), but ical models are called chemo-dynamical (Theis et al. 1992; not before z = 2.4 (Pentericci et al. 2001). There is a general Samland et al. 1997; Bekki & Shioya 1998; Berczik 1999; trend of early type galaxies being older and having higher peak Williams & Nelson 2001), if they include different stellar popu- star formation rates than late type galaxies (Sandage 1986). lations, a multi-phase ISM, and an interaction network that de- These general star formation histories seem to be overlapped scribes the mass, momentum and energy transfer between these with star formation bursts, triggered by infalling material or in- components. teractions with other galaxies. The aim of the present work is to investigate how a large Since we can observe galactic sub-structures down to the disk galaxy forms inside a growing dark halo in a realistic stellar scale only in our Galaxy, the Milky Way observations are ΛCDM cosmogony. We present the results of a new three- a cornerstone for the understanding of the formation and evo- dimensional model, including dark matter, stars, and a multi- lution of all spiral galaxies. From studies of Galactic stars we phase ISM. The mass infall at the boundaries of the simulated know that the halo is old and that the disk and bulge contain a volume is taken from cosmological simulations (see VIRGO- mixture of old and young stars. This raises the question in what GIF-project; Kauffmann et al. 1999). We use a total dark matter sequence the galactic components halo, bulge and disk formed, 12 11 mass of 1.5 × 10 M, a total baryonic mass of 3 × 10 M, and what was the star formation history. Determinations of a spin parameter λ = 0.05, and an angular momentum pro- the star formation history of the Milky Way and other nearby file similar to the universal profile found by Bullock et al.
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